With an increasing demand for down-sizing in shape, high-speed processing and larger capacitance of electronic instruments such as cellular telephone and personal computer, capacitors smaller in size and greater in capacitance are needed.
Among down-sized capacitors, tantalum capacitors are preferably used because it has a large capacitance for the apparent size and exhibits good performance.
In this tantalum capacitor, a sintered body of powdered tantalum is generally used for the anode body. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the amount of the powder for capacitors used or to use a sintered body with a surface area increased by pulverizing the powdered tantalum.
According to the former method of increasing the amount of the powder for capacitors used, the capacitor size is necessarily enlarged and the requirement for downsizing cannot be satisfied. On the other hand, in the latter method of pulverizing the powdered tantalum to increase the surface area, the leakage current value (hereinafter referred to as “LC”) increases although the capacitor may be increased in the capacitance. Thus, the both methods have problems in performance.
As an approach for solving these problems, a capacitor has been studied which uses a sintered body of a powdered material having a dielectric constant larger than the tantalum. As such a material having a larger dielectric constant, niobium is known.
Assuming that the leakage current value of a sintered body after electrolytic oxidization measured at a voltage of 70% of the electrolytic voltage after a lapse of 3 minutes is defined as an LC value, the LC value of a sintered body using a high-capacitance powdered tantalum having a product CV of the capacitance and the electrolytic voltage of 40,000 per 1 g is usually around 30 μA/g. Whereas, the LC value of a sintered body using conventional powdered niobium is 100 times or more larger than that.
Furthermore, the conventional capacitors using a such a sintered body as this material have another problem in that the reliability is low due to deterioration of the capacitance, and cannot be used in practice.
As such, the capacitor using a niobium sintered body disadvantageously allows the electric instrument to greatly dissipate the electric power in vain due to the LC failure. Furthermore, not a small number of capacitors deteriorate and decrease in capacitance after the use for a few years, failing in gaining sufficiently high reliability, and therefore, these cannot take the place of conventional tantalum capacitors in practice.
On the other hand, tantalum capacitors are also demanded to have larger capacitance, lower LC value and smaller deterioration in the capacitance.